The present invention relates to an arrangement for correcting the way in which print hammers strike. In particular it is intended to correct for mis-alignment in impact printers which results from variations in the speed of the character support.
Modern impact printers have a character support (a rotary drum or a linear character support) which moves cyclically in front of a text or sheet support such that all the characters are made available at each print position.
At each print position there is a corresponding strike member which releases a print hammer at the required time, that is, when the character to be printed is at the appropriate print position.
This release is caused by the action on the hammer of an actuator, normally an electromagnetic or electrodynamic actuator, which is operated by a strike amplifier.
Although their performance is similar from the point of view of printing speed, printers having a linear support are preferred to drum printers at the present time because of the better standard of print they give.
Among linear supports, endless type-carrying belts are well known. Such a belt is described, for example, in French Pat. No. 1,602,392 which was filed on Nov. 19, 1968 corresponding to U.S. Pat. No. 3,844,211. This belt is a metal one, made of steel, which is tensioned over two pulleys having parallel axes of rotation and which is provided on its top edge with N type-carrying fingers in the form of flexible tongues, the entirety of these N fingers consisting of a whole number of identical sets of different characters.
The belt passes horizontally before the text support and the group of strike members with a uniform linear motion. Along the bottom edge it has a line of N synchronization holes, each of these holes being associated with one and only one of the N characters. At a level different from that of the N holes is situated another synchronizing hole known as the start-of-belt hole which corresponds to one particular character among the N characters, such as the first in the series of characters making up one of the aforementioned sets.
With the line of N holes is associated a first sensor (magnetic or opto-electronic) which is connected to the framework of the printer and which gives a signal each time a hole, and thus a character, passes in front of it. This signal is amplified and shaped and is transmitted to the logic unit which controls the printing of the printer.
A second sensor, which is likewise connected to the framework of the printer, is associated with the "start-of-belt" hole and gives a signal each time the latter passes in front of it. The signal, once amplified and shaped, is also transmitted to the logic unit of the printer.
Combining the N signals transmitted by the first sensor with the signal from the second sensor makes it possible to locate any of the characters with certainty, that is, it is always known which character-carrying finger is passing in front of a particular hammer.
To make allowance for the response time of the strike members, it is necessary for a certain length of time to elapse between the moment at which each synchronizing hole is picked up and the moment at which the hammer actually strikes the character which corresponds to this hole. To this end:
1. the axis of symmetry of each synchronizing hole is equidistant from pairs of successive fingers; and
2. each strike amplifier has in series with it a monostable circuit termed a "delay" circuit.
When a hole has been picked up by the logic unit of the printer and the corresponding character needs to be struck by the hammer which is in coincidence with the character, the logic unit gives an order to the monostable circuit which then emits a so-called delay pulse of adjustable duration td. When this pulse drops to logic zero, the strike amplifier associated with the hammer provides the hammer with the energy needed for it to strike. Duration td depends on the strike time, i.e., the interval which elapses between the time at which the logic unit of the printer gives the order to strike and the time at which the hammer effectively strikes the character.
Duration td of the delay pulse is adjusted so that the strike time is the same for all the hammers (this operation is termed "alignment of the hammers"), and this duration is a function of the performance required from the printer and thus of the speed of the belt.
The pulleys on which the belt is mounted are driven by motors having a rotational speed subject to variations in the supply or mains voltage. If the delay times (and thus the strike times) have been adjusted for only a single belt speed, any variation in the belt speed upsets the striking action and results in mis-alignment of the print. The characters are struck too early or too late and this detracts from the quality of the print.
To remedy this state of affairs, there are a number of solutions in current use. When the drive is by means of synchronous motors, an associated inverter is used. When the motors are DC, they are regulated directly.
However, these solutions are cumbersome and expensive and still allow variations in speed which remain troublesome.
The present invention makes it possible to overcome these drawbacks by advancing or retarding the time at which the monostable delay circuit produces the delay pulse depending on whether the speed decreases or increases. The delay time is increased or reduced in this way, and the strike time of the hammers is adjusted in the most favorable way as a function of the changes in the speed of the belt.
A correcting arrangement made in accordance with the invention can be used with an impact printer which has a linear character support. The support moves at a nominal speed V past a text or sheet support and is provided with the same number of synchronizing reference-points as there are characters. A fixed detector which detects the reference-points is arranged close to the path followed by them and produces a so-called synchronizing pulse each time a reference-point passes in front of it. The printer also has a strike hammer responsive to a synchronizing pulse for moving from a rest position to a strike position in contact with a predetermined character on the sheet.
When used with such a printer, a preferred form of the invention would include means for generating a deviation speed signal representing the deviation of the actual carrier speed from the nominal speed. Strike means are provided for moving the strike hammer from the rest position to the strike position in response to a strike pulse. The strike means typically could include a monostable multivibrator which pulses a strike amplifier that releases the strike hammer. Delay means apart from the multivibrator generate the strike pulse after a delay time period proportional to the value of the deviation speed signal, said delay time period commencing with the receipt of the synchronizing pulse by the delay means. By using this apparatus, the strike hammer will strike the character at the proper time irrespective of carrier speed variations.
In accordance with other more detailed features of the invention, the correcting arrangement comprises:
a second fixed detector (C) for detecting the reference points located at a distance from the first detector equal to an odd number of half inter-character spaces. The second fixed detector generates a check pulse each time a reference point passes in front of it;
a logic correcting circuit associated with the two detectors comprising:
a first time-base clock;
a circuit for measuring overall strike time;
a subtractor circuit;
a register;
a downwards counter; and
a second time-base clock associated with the downwards counter.
Chronologically, the logic correcting circuit operates as follows:
the measuring circuit receives at a first input the synchronizing pulse, at a second input the check pulse corresponding to the same reference point and at a third input the pulses supplied by the first time-base clock;
during the time interval separating the synchronizing and check pulses, the logic correcting circuit calculates the number N of pulses equivalent to the actual strike time relative to the actual speed of the character support, this actual strike time being equal to the product of N multiplied by h, h being the period of the pulses supplied by the second time-base;
finally the logic correcting circuit transmits an actual speed signal corresponding to the number N to the subtractor circuit as soon as the check pulse is received.
The subtractor circuit calculates the difference N between N and N', which is the number of pulses equivalent to the strike time relative to the nominal speed of the said support and is equal to the product of N' multiplied by h. In other words, N' corresponds to the nominal speed of the support.
This difference N is a deviation signal which is transmitted to the register and then to the downwards counter. The downwards counter receives at a first input the synchronizing pulse and at a second input the pulses of period h from the second time-base, and allows the synchronizing pulse to be transmitted to the delay generator after a time tr which is equal to the product of .DELTA.N multiplied by h.